When chip products are prepared from starch-containing vegetables, excessive sugar levels in the vegetable can lead to an unacceptable dark colouration of the chips on cooking, due to non-enzymatic browning arising from caramelisation of sugars on the surface of the chip products and Maillard browning due to reaction of sugars and amino acids.
Various methods of extracting excess sugar from vegetable slices before cooking have been used to overcome this problem. For example, U.S. Pat. No. 4,743,455 to Nichols discloses a method for blanching raw potato slices before chip production.
Such methods may remove valuable nutrients along with excess sugar and may affect the texture or quality of the final product.
It is preferable to use vegetables having sugar levels which will not cause a discoloration problem.
However, even vegetables which have satisfactory sugar levels at harvest may undergo low-temperature sweetening or chill-sweetening when stored at low temperatures, due to conversion of starch to free sugars, resulting in unacceptably high sugar levels after storage. Vegetables subject to chill-sweetening include potatoes, carrots, cassava and sweet potatoes.
Chill-sweetening of a crop such as potatoes can cause substantial losses to growers and processors.
Since chill-sweetening of potatoes becomes significant below 10.degree. C., potatoes are generally stored at 12.degree. C. in the french fry and potato chip industries. At this temperature, however, dry matter loss occurs during storage and steps must be taken to control sprouting, usually by spraying with sprout inhibitors. Different potato cultivars vary widely in their susceptibility to chill-sweetening.
If this susceptibility could be determined, and if cultivars resistant to chill-sweetening could be identified, lower storage temperatures and better identification of batches and selections of crop suitable for storage would be possible.
No suitable method has been available, however, to permit such a determination.
Previous workers had suggested that cold lability of glycolytic enzymes led to decreased sugar metabolism and hence chill-sweetening Pollock, C. J. and Rees, T. (1975) Phytochemistry, vol. 14, p. 613. As already mentioned, this is undesirable due to various reactions of sugars on the product surface during the cooking process.
Examination of the respiratory enzyme activities of chill-sweetening-susceptible and -resistant potato cultivars has, however, shown that these enzymes play little or no role Barichello, V. et al. (1990) Journal of Food Science, vol. 55, p. 1060.
Differential Scanning Calorimetry (DSC) has been used to study the effect of various manipulations on the gelatinisation of starches Stevens, D. J. et al. (1971) Die Starke, vol. 23, p. 8; Wooton, M. et al. (1979) Starch-Starke, vol. 31, p. 201; Wooton, M. et al. (1979) Starch-Starke, vol. 31, p. 262; Donovan, J. W. (1979) Biopolymers, vol. 18, p. 263; Biliaderis, C. G. et al. (1980) Journal of Food Science, vol. 45, p. 1669; Morise, T. J. et al. (1985) "Properties of Water in foods" (eds. Simatos and Multon) p. 211; Lund, D. B. (1983) "Physical Properties of Foods" (eds. Peleg and Bagley). DSC scans of native starch from various species including potatoes have been compared, as well as the effect of chemical modifications of wheat starches on their DSC profiles Wooton et al., page 201, supra, and Wooton et al., page 262, supra.
Donovan supra has studied DSC endotherms of purified potato starch granules in the presence of different concentrations of water and has proposed mechanisms for the physico-chemical changes occurring on gelatinisation. Further efforts to characterise the physico-chemical changes of starch gelatinisation have been described by Biliaderis et al supra.
Maurice et al. supra have used DSC to examine the effects of heat treatment at various times, temperatures and water contents on rice and waxy maize starch.
Lund supra has examined the effect on gelatinisation of different proportions of amylose and amylopectin in starch using DSC techniques.
Prior to the present invention, the emphasis in chill-sweetening studies was on enzyme patterns and it was not suspected that starch granule structure and stability was important in this phenomemon.
It was not expected that DSC profiles, which reflect such starch granule characteristics, could be used to indicate the suitability of starch-containing vegetables for chip preparation free of discolouration problems and to provide an indicator of susceptibility to chill-sweetening in starchy vegetables subject to this problem.